“The Spiritual disciplines are those personal and corporate disciplines that promote spiritual growth. They are the habits of devotion and experiential Christianity that have been practiced by the people of God since biblical times.”  Private worship is the track on which spiritual disciplines run. “True spirituality requires one to train at godliness in one’s walk with the Lord.” 
The purpose of private worship is to attain godliness. It is a striving to be Christlike and holy. God has revealed himself in two primary ways: Generally in nature, and specifically in Scripture. General revelation is God’s work in creation that most certainly points to His existence as creator (Psalm 19:1; Romans 1:20). While the Apostle Paul makes clear that this revelation is enough for man to be condemned when he does not honor God in the way that He requires, it is by the grace of God that He has chosen to reveal himself to us otherwise in special revelation, namely through the Bible. Therefore, private worship is most importantly about understanding, recognizing, and rejoicing in God’s nature, character, law, decrees, and conditions upon which mankind is redeemed. Wayne Grudem explains:
It must be emphasized that Scripture nowhere indicates that people can know the gospel, or know the way of salvation, through… general revelation. They may know that God exists, that he is their Creator, that they owe him obedience, and that they have sinned against him. The existence of systems of sacrifice in primitive religions throughout history attests to the fact that these things can be clearly known by people apart from the Bible. The repeated occurrences of the “rain and fruitful seasons” mentioned in Acts 14:17 may even lead some people to reason that God is not only holy and righteous but also loving and forgiving. But how the holiness and justice of God can ever be reconciled with his willingness to forgive sins is a mystery that has never been solved by any religion apart from the Bible. Nor does the Bible give us any hope that it ever can be discovered apart from specific revelation from God. It is the great wonder of our redemption that God himself has provided the way of salvation by sending his own Son, who is both God and man, to be our representative and bear the penalty for our sins, thus combining the justice and love of God in one infinitely wise and amazingly gracious act. This fact, which seems commonplace to the Christian ear, should not lose its wonder for us: it could never have been conceived by man alone apart from God’s special, verbal revelation. 
Therefore, central to daily private worship is the reading, study, and meditation on the Word of God for the purpose of conceiving of and believing in who God is, and what He has and will continue to do.
As with all biblical paradoxes, it is important to acknowledge the both/and nature of private worship. Since private worship is for the purpose of godliness, man is responsible to take faithful action to cultivate a work that only God can do. In other words, godliness is both man’s responsibility, and God’s sovereign work. Man attains godliness through spiritual disciplines. God grants godliness as a divine gift of grace. This is God’s continuous work in the lives of believers called sanctification (John 17:17; 1 Thessalonians 5:23; Hebrews 2:11). Christian believers must strive for godliness through the spiritual disciplines given to us by God, and trust in the absolute sovereignty and goodness of God, knowing that the work is all of Him. Every man is responsible for his life and actions, but God will give the Christian new affections, desires, and longings that please Him.
Private worship is invaluable for one who wants to truly know God. If a man is to fulfill the most important of all the commandments, to “love the Lord your God with all your heart and with all your soul and with all your mind” (Matthew 22:37-38), he must first know not only the letter, but also the spirit of this command. Likewise, the Bible teaches that, “The heart is deceitful above all things, and desperately sick” (Jeremiah 17:9). If one does not engage in daily, private worship, this sick and deceitful heart will be his guide. Private worship is the means by which God does His work of sanctification in our lives. This is certainly not the only means, but it is a primary means:
Have you ever met someone who has a phenomenal knowledge of the Bible but has a distinct lack of grace in their life? They can reel off a string of verses they have memorised [sic] and can explain any biblical doctrine you ask them about, but the life doesn’t seem to match up to it. The reason for this may be that the Word of God is in their head but not in their hearts. Moses told the Israelites that “the word is very near you: it is in your mouth and in your heart so that you may obey it” (Deuteronomy 30:14), and Jesus spoke about His Word abiding in us (John 15:7). [Private worship] provides the opportunity for the Word to penetrate our hearts and transform our lives. 
It must be mentioned that Bible reading and study is not the only aspect of private worship, but it is of utmost importance. Our ability to pray and sing and focus our hearts and minds on God is all informed by the words that God has inspired for our benefit. “No spiritual discipline is more important than the intake of God’s Word. Nothing can substitute for it. There simply is no healthy Christian life apart from a diet of the milk and meat of Scripture. .. if we would know God and be godly, we must know the Word of God — intimately.”  The Bible is filled with the very words that Peter says, “Angels long to look” (1 Peter 1:12). “Isn’t it tragic that where angels long to look we can’t always be bothered to glance! Perhaps we have begun to take God’s blessings for granted. If you come to your [private worship] feeling weary and wanting to get [it] over as quickly as possible, stop and think about the privilege that you have been given! You are able to meet with your creator, you are invited to encounter Almighty God, to hear from Him and worship Him. What better reason could there possibly be to set the necessary time aside?” 
1. Donald S. Whitney, Spiritual Disciplines for the Christian Life (Colorado Springs: NavPress, 1991), 16-17.
2. Earl D. Radmacher, Ronald Barclay Allen and H. Wayne House, Nelson’s New Illustrated Bible Commentary (Nashville: T. Nelson Publishers, 1999), 1 Ti 4:7.
3. Wayne A. Grudem, Systematic Theology : An Introduction to Biblical Doctrine (Leicester, England; Grand Rapids, Mich.: Inter-Varsity Press; Zondervan Pub. House, 1994), 123.
4. Simon J. Robinson, Improving Your Quiet Time (Leominster: Day One Publications, 1998), 21-22.
5. Whitney, 28.
6. Whitney, 23-24.